1. Field of the Invention
The invention relates to an automatic focusing device for a photographic enlarger and more particularly to such an automatic focusing device wherein, once an enlarging lens has been set manually to its infocus condition, the enlarging lens is automatically shifted to its in-focus position in response to a change in distance between the light source head and the easel plane or the plane of the printing paper.
2. Description of the Prior Art
Heretofore, various devices of the above mentioned type have been known. One of them is shown in Japanese laid-open patent application No. 57-100415. With the device of the Japanese laid-open patent application, after the distance between a film and an easel on which a printing paper is placed is determined to provide an appropriate enlarging magnification and the focusing of the enlarging lens is manually adjusted, the distance from the film to the enlarging lens is calculated and memorized in accordance with the distance between the film and the easel and the focal length of the enlarging lens. Thus, an initial position setting is completed. When the distance between the film and the easel is changed for changing the enlarging magnification after the completion of such an initial position setting, the device calculates a new film enlarging-lens distance, from the amount of the change of the film-to-easel distance and the distance relating to the aforementioned initial position. Then, the device shifts the enlarging lens to a position commensurate with the calculated distance.
However, the known device has a disadvantage in that the distance between the film and the printing paper (the easel) can not be known and the focusing can not be adjusted automatically in the case where the easel is removed and the printing paper is placed on the floor for large magnification enlarging. This is because the new position of the enlarging lens is determined in accordance with the amount of the change of the film-to-easel distance.